The present invention relates to a method and an apparatus for obtaining distance and velocity information from multiple objects using frequency modulated, continuous wave (FMCW) radar, especially for use in automotive adaptive cruise controls (ACC).
The FMCW radar is suitable for the distance measurement of plural objects with small relative velocities. However, in an application like automotive ACC, there are plural objects and the relative velocity changes widely, e.g. from xe2x88x92200 km/h to +200 km/h. The plural objects and the wide velocity range make it difficult to determine the exact pairing of the beat frequencies in each ramp, corresponding to a certain object. And after a pair is correctly selected, the wide velocity range results in an ambiguity between the distance and relative velocities of the objects. Several methods to overcome this problem have been proposed, but such methods are complicated and not practical.
For example, if only one target can be illuminated at a time, alternating frequency modulation ramps with opposite slopes can be used. In certain multiple target situations, a third frequency ramp is used, generally with zero slope and then finally a fourth sequence may be necessary to remove the doubt. Also, the distance and relative velocity of objects can be determined by using a complex look-up table, which has the information on the beat frequencies in each ramp.
The present invention concerns an apparatus and method utilizing an algorithm based upon a two chirp frequency sweep with small slope-difference. Since the difference is small, the correct pairing of the beat signals in each ramp is uniquely determined. The distance information can be obtained by automatically eliminating the Doppler frequency. Therefore, the distance and velocity information for multiple objects can be calculated without ambiguity.
The method according to the present invention comprises the steps of: a. generating a continuous wave radar signal; b. modulating the continuous wave radar signal with a pair of chirp frequency sweeps having slightly different slopes to generate a frequency modulated continuous wave (FMCW) radar signal; c. transmitting the FMCW radar signal toward a pair of objects; d. receiving a reflected radar signal from each of the objects; e. obtaining the beat signals by mixing the transmitted and reflected radar signals; f. applying the Fourier Transform to the beat signals and determining the correct pair of the peak frequency of the beat signals in each ramp, corresponding to a certain object; g. calculating the distance from the pair by eliminating the velocity; h. calculating the velocity by using the distance information; and i. indicating a distance and/or relative velocity to each of the objects.
The apparatus, according to the present invention, for detecting multiple objects using frequency modulated continuous wave radar includes: a modulator for generating two chirp signals, the two sweeps having slightly different slopes; a RF oscillator for generating a continuous wave radar signal frequency-modulated by the modulator; a directional coupler dividing the radar signal; a transmitting antenna connected to the coupler for transmitting the divided frequency modulated continuous wave radar signal; a receiving antenna for receiving a reflected radar signal representing a portion of the transmitted radar signal reflected from each of plural objects; a mixer and a low/band pass filter connected to the coupler and to the receiving antenna and responsive to the transmitted radar signal and the reflected radar signal for generating a beat signal including the information of all the objects for each of the two sweeps; and a Low/band pass filter for filtering the beat signal and a signal processor connected to the filter and responsive to the beat signals for applying the Fourier Transform, usually FFT (Fast Fourier Transform), calculating the distance and velocity for each object, and indicating a distance to each of the objects.
The present invention is proposed for multiple object detection in automotive FMCW radars. Such radars include ACC (adaptive cruise control) radar, Stop and Go, collision avoidance radar, side-object detection radar, etc. The algorithm according to the present invention can provide the distance and relative velocity for multiple objects. This algorithm is simple and fast, and suitable for automotive application, in which the update time is less than 100 msec.